1. Field of the Invention
The present invention relates to resistive sensor, particularly to resistive cholesterol sensor based on conductivity change of the conductive polymer.
2. Description of the Prior Art
Cholesterol detection draws a lot of attention because the abnormal cholesterol level may indicate clinical disorders such as heart disease, hypertension, arteriosclerosis, and cerebral thrombosis etc. According to the report published by World Health Organization, 17.3 million people were estimated that died in cardiovascular diseases in 2008 globally and the death toll keeps increasing annually. It is crucial to monitor cholesterol level in blood for estimating the potential risk of cardiovascular disease and preventing deaths from the disease. Many researches have been devoted in the development of cholesterol sensors.
There is an increasing demand on for cheap and disposable electronic cholesterol sensors for the point-of-care health monitoring. Electronic biosensors, such as electrochemical, field-effect-transistor-based, or resistive biosensors are cheap and small thanks to the mature microfabrication techniques. Therefore, it is possible to develop miniaturized and disposable electronic microsensors with high sensitivity and fast response, which can detect cholesterol level in typical physiological environment for routine monitoring.
Most electrochemical sensors detect cholesterol by voltammetry, in which, cholesterol oxidase was immobilized on electrode and oxidize cholesterol into cholestenone.
The reduced cholesterol oxidase will then be oxidized via the electrode and therefore the catalysis of the enzyme reaction can continue. Different materials were used for the electrode to enhance charge transfer rate or to immobilize cholesterol oxidase (ChOx) more efficiently.
Multiwall carbon nanotubes (MWCNT), conducting polymer-MWCNT composite, MWCNT/Screen Printed Carbon Electrode (SPCE), conducting polymer, graphene, or nano structured metal oxides such as ZnO nanorods (NRs), ZnO nano sphere, anatase-titanium dioxide and SnO2 nanoparticles were reported for electrode materials which can improve the charge transfer rate or ChOx immobilization. ChOx-immobilized ZnO NR-gated field-effect-transistor (FET) was also reported to be able to sensitively detect cholesterol.
However, compared to resistive biosensors, electrochemical sensors and FET-based sensors are more complicated in sensor design or the measurement system. The resistive type of sensors exhibits conductivity change, which is directly proportional to the cholesterol level, and therefore is much simpler for device fabrication and signal measurements.